Inrunner Motor Torque: What To Check

Quick Answer

A brushless inrunner motor's torque output depends on stator size, winding configuration, pole count, and operating current. A typical 22 mm inrunner produces around 0.2–0.5 N·m at rated load; a 42 mm version can reach 1.5–4 N·m. Torque falls linearly as speed rises. Before selecting a unit, verify the torque-speed curve at your operating point, confirm the driver's current limit matches the motor's current rated, and check thermal derating under sustained load. Mismatching any of these parameters leads to overheating, shortened lifespan, or mechanical failure before the motor reaches its design life.

Introduction

You spec a brushless inrunner, the driver delivers current, and the rotor still stalls under load. This gap between expected and actual torque shows up in countless procurement conversations. The root cause is rarely the motor itself. More often, it is a mismatch between the motor's torque-speed curve, the driver's current ceiling, and the real mechanical load profile. Understanding how these three elements interact saves you from ordering the wrong size, over-specifying on cost, or burning a unit within the first week of operation.

1. How Torque and Speed Trade Off in an Inrunner

Think of the inrunner's torque-speed curve as a sliding scale. At stall (zero), the motor produces its maximum torque. As the rotor accelerates, back-EMF builds and available torque drops in a near-linear fashion until it hits zero at no-load speed. Two practical consequences follow. First, your operating point sits somewhere on that slope, and the torque available there is all the load actually sees. Second, sustained operation near the stall region generates heat the stator was not designed to dissipate. Always confirm with the supplier where your duty cycle places you on the curve, and request thermal derating data if you expect more than 30 seconds of high-torque operation in any cycle.

2. Specifications That Actually Determine Torque Output

Before you compare two inrunner datasheets, check these parameters in sequence:

Stator outer diameter and core height: a larger air gap produces more torque at the same current. A 30 mm motor will out-pull a 22 mm unit.

Winding type (concentric vs distributed): affects inductance, which changes how much current the driver can push before resonance limits kick in.

Pole count: more poles lower no-load speed for a given voltage but raise low-speed torque resolution.

Rated vs peak current: torque scales with current up to the rated limit. Exceeding it triggers thermal protection or demagnetisation risk.

Driver current limit: the driver caps actual torque. A motor rated for 20 A is effectively a 10 A motor if the driver is set to 10 A.

Cross-reference the driver spec sheet with the motor spec sheet before placing an order.servomotor selectionbecomes straightforward once both documents align.

Values vary by manufacturer and winding. Request the full torque-speed curve before finalising yourtorque requirements .

3. Where Torque Selection Goes Wrong

The most common failure pattern is sizing by stall torque alone. A motor that delivers 3 N·m at stall may produce only 1 N·m at 50 % of no-load speed. If your application runs continuously at that speed, the effective torque is one-third of what the datasheet headline suggests. A second frequent error: assuming the driver and motor are matched. In many procurement cases, the driver ships with a lower default current limit than the motor's rating, quietly capping usable torque. Third, ignoring gear ratios. A high-ratio gearbox multiplies torque but cuts speed; verify the combined output meets yourmotion control applicationsspec, not just the motor's bare numbers. When in doubt, ask the supplier for a combined system torque curve rather than relying on a single component's datasheet.

4. Practical Questions Before You Order

What torque should I expect from a standard 30 mm brushless inrunner?

A 3010 or 3012 inrunner typically delivers 0.5–1.2 N·m at rated current with a 5,000–15,000 RPM no-load speed. Exact figures depend on winding and pole count. Request the torque-speed curve at your operating voltage before comparing two models side by side.

Can I increase torque by simply raising the driver current?

Not beyond the motor's rated current without risk. Short peak-current bursts are acceptable within the manufacturer's thermal derating window. Sustained over-current raises winding temperature, degrades insulation, and can demagnetise the permanent magnets. Always check the thermal data sheet for the safe peak duration before setting driver limits.

How do I know if my driver and motor are properly matched?

Run three checks. The driver's continuous current rating should meet or exceed the motor's rated current. The driver's voltage window must cover the motor's operating voltage. The driver's current-limit setting should be programmed to the motor's rated value, not left at factory default. If any check fails, your available torque is lower than the datasheet implies.

5. Choosing the Right Inrunner for Your Application

Torque on a brushless inrunner is not a single number. It is a curve, a current limit, a thermal boundary, and a system interaction. Before you commit to an order, align the motor spec, driver spec, and load profile in one document. If you want a second pair of eyes on your selection,kpower servoengineering can review your torque requirements, duty cycle, and driver configuration, and confirm the fit before you place a purchase order.